Stock Car Suspension - Moment Center Technology

1/13Note the difference in each car's left front wheel. Photo by Jeff Huneycutt

Well it has finally happened. The stock car racing world has embraced the importance of front moment center location and design. Everywhere I turn racers, car builders, engineers, and racing schools are talking about moment centers (also referred to as roll center) and where it should be located. This is fantastic news because it means we can now talk the same language.

The growth of information about the front moment center during the last 10 years has helped us to understand the way in which this invisible point controls the front dynamics of our racecar. We first published information about the front moment center in the May 1998 issue of CIRCLE TRACK. At that time, much of the industry did not understand exactly how the moment center (MC) worked and many thought that it was just not important.

2/13The front moment center location is the result using the intersections of lines extended from the centers of rotation of the control arms to form the instant centers. The MC is at the intersection of lines drawn from the left and right ICs to the centers of the contact patches of the tires.

Since then, we have defined exactly what the MC does and where it should be located based on research and input from racers across the globe. We have also coined the word for that point that better defines its role in the cars dynamics-moment center. There are several roll centers in our racecars which includes the kinematic roll center, which is based on the motion of the chassis, and the dynamic roll center which is the bottom of the moment arm in every double A-arm suspension. Because the moment center is seldom the point about which the chassis rolls, we should stop referring to it that way.

Definition Of The Moment Center The MC in the front suspension is the bottom of the moment arm for that suspension system. The moment arm is like a leverage bar that creates the rolling force in a double A-arm suspension. The top of the moment arm is the center of gravity (CG) of the sprung mass (the weight of the front of the car minus the unsprung components such as the wheels, tires, brakes, rearend, etc.) and the bottom is the MC.

Myths About Moment CentersHere are some popular myths concerning the moment centers that have now been disproved.

1. The front moment center is at the center of the car.

Wrong The front moment center is rarely at the centerline of the racecar. It also moves laterally as the car dives and rolls, some designs moving to the left and some to the right.

3/13When considering the angle of the control arms, never rely on the angle of the tubing that connects the ball joint and the chassis mount. The center of rotation of the ball joint is almost never directly in line with the center of the tubing. If we lay an angle finder on the tubing, the angle we read will not represent the true "arm angle" which is the angle of the line between the centers of rotation.

2. The front moment center location is not important to the dynamics of the chassis.

Wrong The front moment center location is critical to the dynamics as we will show. Because it is the bottom of the moment arm, its location dictates the length of the moment arm and therefore the amount of force that will initiate roll in the chassis.

3. When we change front control arm angles, we are really affecting the camber change characteristics and/or the jacking forces on the instant centers and the changes to the handling of the car are related to that, not the new moment center location.

Wrong We can move the MC while affecting the camber change characteristics and the "jacking effects" very little and see a drastic change in the handling. This serves to disprove those notions.

4. I can draw out my moment center on the garage floor or on paper to find its location.

Wrong The reason why the static location, the one you draw out, is not really important is because the MC moves as the chassis dives and rolls going into and through the turns. Where it ends up is the most important aspect of MC design because it affects the turn dynamics where we desire our performance. It is very difficult to draw this dynamic location without a seriously complicated drafting software program.

Explanation Of Moment CenterHere is an explanation of how the front MC location is derived and what affects its location. The sketches are showing both the static and dynamic locations. Moment center geometry software will help you determine the dynamic location for your MC.

4/13A model was constructed and control arms were mounted on "spindles" and a "chassis." This was a two dimensional representation of a stock car front suspension. The result was a better understanding of how the imaginary point called the MC related to the dynamics of the front of our cars.

On each side of the car we have upper and lower control arms, in a double A-arm suspension system, and those have pivot points at each end being a ball joint at the spindle and bushings or heim joints at the chassis mounts. If we draw a line through the upper control arm pivot points and again through the lower control arm pivots, these two lines will intersect at a point called an instant center (IC). For most stock cars, these IC intersections lie to the chassis side of the spindle. Each set of control arms on each side of the car has their own IC. If we also draw a line from each IC to the corresponding center of contact patch of the tire on the same side as the control arms we used to create the IC, then the intersection of the two lines from the left and right ICs defines the moment center location.

A Simple ExperimentIt is very difficult for us to understand the importance of an invisible point and how it could possibly be important to the dynamics of the front suspension. We generally think in terms of hard points that we can put a bolt through and that are attached to the chassis. The MC is not directly connected to the chassis and has no bolt through it. So, some years ago while I was trying to understand how the MC really worked, I decided to build a model to find out exactly what influence the MC had on a double A-arm suspension.

I built a 2-D model of a double A-arm suspension on a board, with spindles, upper and lower control arms, and the "chassis" portion was weighted and supported by springs. I drilled a series of holes vertically along the centerline of the chassis between the control arm mounts to simulate several locations of the CG of the "car." I also had the ability to change the arm angles so that I could create different locations for the MC.

5/13The two forces that are applied to the CG are the centrifugal force, caused by the change in direction when traveling around a radius, and the ever present force of gravity. In physics, when two forces are simultaneously applied to a point, they combine into a net resultant force. It is the magnitude and direction of this force that we need to know about in order to understand how the MC lateral location affects the dynamics of the suspension.

I began at the top CG hole and attached a string and pulled laterally to the right on the string. The "chassis" would roll to the right each time as I moved down from hole to hole. When I reached the hole that represented the MC, my suspension would lock up and would not roll. As I proceeded down below the MC, the "chassis" would then roll to the left because the moment arm was now inverted.

I changed the MC location several times, and each time when I put the CG nail in the hole that represented the MC, the suspension locked up. That told me that the MC was indeed the bottom of the moment arm and when the MC is in the same location as the center of gravity, there is no moment arm and therefore no lever arm to roll the chassis.

The Industry Begins To Understand Moment CentersOver the past few years, hundreds of racers, as well as numerous racecar builders around the country have experimented with MC location and design and found the correlation between the MC location and front end dynamics to be significant. Front suspension efficiency, as well as camber change characteristics, all depend on the MC height and width. To understand how the lateral MC location affects the dynamics of the car, we need to understand how the forces are applied to the center of gravity and the MC.

Forces And Moment ArmsThe true moment arm that tries to roll the suspension is represented as a line lying at a right angle between the MC and the resultant force line. This is called the effective moment arm. We can see how, with the resultant force pointed down and to the right from the CG that as the MC is located farther to the right, the effective moment arm becomes shorter.

The MC has basically two locations that we can easily calculate, static and dynamic. Static is where the MC is located when the car is at ride height and standing still. The dynamic location is where the MC moves to as the car dives and rolls through the turns. Because the control arms move as the car rolls and dives, the instant centers also move. This causes the MC to move along with the instant centers and in most cases, the MC moves to the right in a left turning circle track racecar.

6/13As the MC moves to the right to the eventual dynamic location, the effective moment arm becomes shorter because of the direction of the resultant force.

Control Arm Angles vs. MC location ControlArm angles are measured in degrees from horizontal. Therefore level would be zero degrees and straight up would be 90 degrees. The lower control arm angles largely control the amount of lateral movement of the front MC and the upper control arm angles mostly control the lateral location of the two MCs, static, and dynamic.

Excessive lower control arm angles cause the front MC to move a greater distance as the car goes through the turns. Increasing or decreasing one or both of the upper control arm angles moves the MC side to side. Low or reverse (the chassis mount being higher than the ball joint) angles in the upper control arms cause excessive camber change in the front wheels, especially in the right front wheel where it affects the handling the most.

With the newer setups such as the big bar and soft springs (BBSS), we need to re-address the issue of camber change versus upper control arm angles. These setups cause much more dive and much less roll. The upper control arm angles will need to be less within the possibilities of a good MC location. The initial cambers for both the left and right tires need to change due to the excessive camber change associated with a lot of dive. The RF static camber must decrease and the LF static camber must increase.

7/13Measuring stagger and taking tire temps are important but will mean little if you don't know your moment center location. Photo by Todd Ridgeway

Rules About MC LocationAs we begin to design our car's front suspension, we need to determine where the MC should be located according to our type of racing and what kind of racetrack we will run. Here are a few general rules that we need to follow:

1. The farther left the MC is located, the longer the effective moment arm and the more efficient the front suspension will be meaning it will want to roll more. A more left location is proper for low banked asphalt racetracks and dry slick dirt racetracks, and cars with a low CG.

2. The farther right the MC is located, the less efficient the front suspension will be meaning it will be stiffer and want to roll less. A MC that is located farther to the right of the centerline of the car will be good for all tracks that are high banked including dirt and asphalt tracks. The higher the banking, the farther right the dynamic MC should be located. Because of the higher amount of downforce caused by the banking, we need the front to be somewhat stiffer to resist excessive dive on entry and in the middle of the turns.

8/13When the right upper control arm has more angle than the left upper control arm, the MC is usually located more to the right of the centerline of the car. The upper control arm angles tend to control the position, left and right, of the MC. The opposite is true if the upper angles are reversed with the left being more than the right.

3. The amount of MC movement from static to dynamic locations that the car needs depends on how the racetrack is constructed. A track that has consistent banking throughout the turns requires a MC design where there is very little movement from the static to the dynamic location. Tracks that are constructed where the banking going in and coming off the corners is far less than the banking in the middle of the turns may require a MC design that incorporates a greater amount of movement of the MC from static to dynamic locations. This is common in dirt track racing where corner entry must be enhanced aided by a MC location farther left.

As the car turns into the corners, the MC can be designed to start out statically farther to the left and that is more suited to the lower banking. Then as the car moves through the middle where the banking has increased, the MC will then move to the right and that location is better suited to the higher amount of downforce caused by the high banking.

4. The lateral location of the MC range is dependant on the height of the CG of the racecar as well as the track width of the car. A modified asphalt stock car with its low CG can tolerate a MC that is farther left than say a stock class car where the ride height is above 5 inches and the CG is above 18 inches. The narrower the car, the greater the effect is on lateral location of the MC. A narrow track width (distance from center to center of the tires) results in a greater tendency for the front end to want to roll. Narrow stock cars should be designed with a MC that is farther to the right than wider cars.

9/13This chart shows suggested dynamic MC locations for various types of asphalt circle track racecars. This information comes from research and feedback from teams.

5. The circle track stock car wants zero camber change, relative to the racing surface, in the right front wheel after the car dives and rolls through the turns. The right upper control arm angle mostly affects the RF camber change characteristics. When designing our MC location, we need to find the correct right upper control arm angle that will result in the least camber change after the car dives and rolls in the turns. This of course cannot be accomplished when running the BBSS setups where excessive dive will always cause excessive camber change.

The left front wheel will always lose plus or minus two degrees of camber regardless of the upper control arm angle and so we can incorporate changes to the left upper control arm angle to locate the MC range laterally once we have found the optimum angle for the right upper arm for ideal camber change.

6. The entire dynamic MC range for dirt and asphalt stock cars lies between 10 inches to the left or right of the centerline of the car. MCs that lie outside that range will be detrimental to the performance of the car. Shallow upper and excessive lower control arm angles (inside points lower than the ball joints) contribute to MC locations that are outside of the optimum range.

10/13This is a chart showing suggested dynamic MC locations for different types of dirt racecars. This information is definitely in use today and comes as a result of a lot of research and feedback from dirt track teams.

Differences In Handling Caused By MCLocationThe primary reason why two seemingly identical cars will handle differently can often be traced to a front MC that is in a different location from car to car. It doesn't take much of a difference in arm lengths or chassis mounting point heights to cause a pair of cars to experience different handling characteristics.

It was just a few years ago that many professional stock car teams experimented with different lengths of upper control arms to tune the handling while holding to pre-selected spring packages and weight distribution numbers. What they were doing was moving the MC left and right in a trial and error way to try to find the ultimate handling balance for a particular racetrack.

ConclusionToday we know better than to waste time tuning with control arm angles and lengths. We know that there is a combination of spring rates, moment centers, and weight distribution for each car at each racetrack that will make the car balanced and consistent. Knowing the role of the MCs and being willing to make changes so that our MCs are in the right position is one of the very first and most important steps we take to achieve the total handling package.

If you don't know where your MC is located, then you don't know the most important piece of information about your car. It's like not knowing what your timing is, or what tire pressures you are running, or gear ratio. Whether you are a team, a car builder, or engineer, make sure you understand the dynamics of the MC and know where it is located in your car. It's not a theory at this point in racing history, it's a fact, Jack.